Many sensors utilize the magnetic properties of magnetostrictive materials to sense torque. Such materials change their magnetic permeability in response to stress, making them ideal for use in a non-contacting torque sensor arrangement. A magnetic field is passed through the magnetostrictive material. The magnetic field propagation changes as the permeability of the stressed magnetostrictive material changes. This permeability change is measured and related to the torque needed to cause such a stress.
U.S. Pat. No. 3,340,729 issued to Scoppe on Sept. 12, 1967, discloses a torque sensor which has a magnetostrictive layer affixed to a nonmagnetic shaft. This patent contains information concerning basic properties of torque sensing using magnetostrictive materials and flux inducing coils. It was intended to overcome drawbacks in the art associated with shafts made of magnetostrictive material. Other torque sensors have improved on this basic concept. U.S. Pat. No. 4,414,855 issued to Iwasaki on Nov. 15, 1983, discloses a torque sensor that uses a strip or strips of magnetostrictive material affixed helically at 45.degree. to a nonmagnetic shaft to reduce inconsistent stresses in the material. U.S. Pat. No. 4,506,554 issued to Blomkvist et al. on Mar. 26, 1985 discloses two zones of slotted magnetostrictive material attached to a shaft.
All of these have inherent shortcomings. Firstly, magnetic flux permeates ferromagnetic materials, such as are typically found in drive shafts and the like. When a magnetostrictive material is bonded to a shaft made of a ferromagnetic material, flux changes may take place in the shaft as well as in the magnetostrictive material. This leads to inaccurate torque measurements. Secondly, if shafts are made from nonferromagnetic materials, the shafts may not meet the requirements for rugged applications. Nonferromagnetic materials are generally weaker and more expensive to produce, as compared to hardened steel shafts. Moreover, affixing a layer of magnetostrictive material to a shaft induces a prestress in the material. The accompanying circuitry may adjust to the prestress, but the range of measurement will be limited since the material reaches saturation under a smaller amount of stress. Even further, the means by which a magnetostrictive material layer is affixed to a shaft effects the sensor's durability and accuracy.
U.S. Pat. Nos. 4,414,856 and 4,503,714 issued to Winterhoff on Nov. 15, 1983, and Mar. 12, 1985, respectively, disclose a magnetostriction torque sensor which uses a shaft made of a magnetostrictive material. Windings on a soft magnetic core induce magnetic flux into the shaft. As the shaft changes its magnetic permeability in response to torque, a measuring head outputs a signal relative to a flux change caused by the changing permeability. U.S. Pat. No. 4,566,338 issued to Fleming et al. on Jan. 28, 1986 discloses another sensor which uses a shaft made of a magnetostrictive material.
These torque sensors display various problems. Shafts of magnetostrictive material are often expensive and difficult to manufacture. Additionally, a bulky piece of magnetostrictive material possesses irregularities which produce inaccurate results.
A magnetic source induces a magnetic flux into the magnetostrictive material of torque sensors of this type, thus providing a non-contacting arrangement for sensing torque. Most of the above-mentioned patents disclose some type of structure for inducing a magnetic flux. Typically a magnetic coil structure having a number of poles surrounds the magnetostrictive material on the shaft. U.S. Pat. Nos. 4,100,794, 4,306,462, and 4,406,168 issued to Meixner on July 18, 1978, Dec. 22, 1981, and Sept. 27, 1983, respectively, disclose circular cores having pole pieces. A primary core induces a magnetic flux into an adjacent shaft. A pair of secondary cores measure changes in magnetic flux caused by magnetic permeability changes in the magnetostrictive material under stress. A low frequency AC source drives the primary core to which the secondary cores are inductively coupled. The secondary cores output to a signal processing circuit which determines any changes in flux between the induced and measured values. U.S. Pat. No. 4,106,334 issued to Studtmann on Aug. 15, 1978 discloses the magnetic core structure of Meixner with a bifilarly wound primary winding. U.S. Pat. No. 4,135,391 issued to Dahle on Jan. 23, 1979 discloses a core arrangement having a primary core arranged perpendicular to the secondary core.
Many problems plague core assemblies such as these. Severe environments inflict much damage to the cores. Oils, vibration, dirt, and temperature greatly reduce their accuracy and lives. The core assemblies also occupy a relatively large amount of space. When attempting to install a torque sensor in an engine or transmission housing, or in any other location having limited area, the physical size of the sensor is extremely critical. Moreover these cores require much raw material. A magnetic core covered by many meters of copper wire represents a significant portion of the total torque sensor cost.
The sheer size of the core structures makes maintaining a consistent air gap between the shaft and the core troublesome. U.S. Pat. No. 4,572,005 issued to Kita on Feb. 25, 1986 and U.S. Pat. No. 4,589,290 issued to Sugiyama et al. on May 20, 1986 disclose mountings which use a bearing structure to maintain a given clearance. However bearings wear out and also cause some damage to the shaft.
The present invention is directed to overcoming one or more of the problems as set forth above.